Browsing by Subject "Quantum Information Theory"

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Open Access
Entanglement in atom-photon systems
(2004) Can, Muhammet Ali
In this work we propose a new principle from the point of view of quantum fluctuations of observables. This new principle can be considered as an operational definition of ME states. Moreover, we show the existence of perfect entangled states of a single “spin-1” particle. We give physical examples related to the photons, and particle physics. We show that a system of 2n identical two-level atoms interacting with n cavity photons manifests entanglement and that the set of entangled states coincides with the so-called SU(2) phase states. In particular, violation of classical realism in terms of Greenberger-Horne-Zeilinger (GHZ) and Clauser-Horne-Shimoni-Holt (GHSH) conditions is proved. We also show that generation of entangled states in the atom-photon systems under consideration strongly depends on the choice of initial conditions In order to obtain maximally robust entangled states we have combined maximum principle with minimum of energy requirement for stabilization, called Mini-max principle. We discuss the generation and monitoring of durable atomic entangled state via Raman-type process, which can be used in the quantum information processing. It is shown that the system of two three-level atoms in Λ configuration in a cavity can evolve to a long-lived maximum entangled state if the Stokes photons vanish from the cavity by means of either leakage or damping. We presented some results based on the application of spherical wave representation to description of quantum properties of multipole radiation generated by atomic transitions. In particular, the angular momentum of photons including the angular momentum entanglement, the quantum phase of photons, and the spatial properties of polarization are discussed.
Open Access
Robust entanglement in atomic systems
(2005) Çakır, Özgür
Various models for generation of robust atomic entangled states and their implementation with current accessible technologies are proposed and worked out. Deterministic creation of long living Bell states with respect to metastable states in three-level Λ type systems is studied. Strong atom-field coupling drives atoms into a transient entangled state followed by an irreversible evolution towards a long-living maximally entangled state featuring robustness against dipole-allowed transitions. First, generation of pairwise atomic entanglement in cavities in ideal case is discussed, extension to multi-party entangled states is made. Observation of photons emitted from the system signals the generation of a Bell state. The interaction of multi-level atoms with body-assisted electro-magnetic field in the presence of dispersing and absorbing media is studied and these results are applied to the description of a pair of Λ type atoms passing by a microsphere. Microspheres give rise to resonances of well defined height and width with easy access to strong and weak coupling regimes for atom-field interaction, thus enabling realization of the proposed scheme of ”robust entanglement of three-level atoms”. Even in realistic settings it is possible to obtain quite high amount of entanglement at spatially well separated distances. Then we focus on steady state entanglement between atomic dipoles. It is shown that two dipoles in free space driven by a classical driving field become entangled in the steady state. The crucial point is that, this entanglement is irrespective of the initial state and may be preserved as long as the engineered system is kept intact. Absorption effects in real cavities are studied, and an input-output relation is formulated in the presence of a source in the cavity. Extraction of non-classical photon states from a cavity is investigated.